The present invention relates generally to radio frequency (RF) receivers, and more particularly to removal of DC offset in a direct downconversion RF receiver.
In many wireless communication formats no information is provided at DC, and DC offset may not directly corrupt received information. Nevertheless, presence of DC offset may reduce the effective dynamic range of the receiver, and potentially can be sufficiently large that signal processing circuitry is unable to distinguish information in a received message. Accordingly, RF receivers often include circuitry for reducing DC offset.
DC offset may arise from a number of sources. These sources include DC offset caused by mixers and DC offset caused by nonidealities in the baseband amplification stage. DC offset from a mixer can arise, for example, due to leakage from one port of a mixer to another. Often DC offset is most pronounced with respect to leakage of a local oscillator signal, and may vary from receiver to receiver due to random variations in the manufacturing process resulting in different parasitic leakage levels. Similarly, in the baseband amplification stage nonidealities in manufacture and variations of component characteristics with temperature may result in introduction of spurious DC components into a received signal.
Complicating efforts to remove DC offsets, DC offset may be dependent upon operation frequency, level of amplification of the received signal, and temperature, and in many instances RF receivers are required to receive signals over a wide range of frequencies, over a wide range of received signal strengths, and over wide temperature ranges. In addition, some communication formats provide for frequency hopping patterns, with little delay between frequency hops. In some such instances, recalibration or selection of different gain settings between frequency hops may be difficult in the allotted time.
In superheterodyne receivers an intermediated frequency signal may be filtered to reduce DC offset. In direct downconversion receivers, however, filtering of intermediate frequency is generally not available as the signal is generally downconverted directly to baseband. Moreover, the filtering of the intermediate frequency signal is often performed using surface acoustic wave (SAW) filters. However, SAW filters are often relatively large and expensive. Eliminating SAW filters from an RF receiver architecture allows for smaller form factors, lower power consumption and a reduced bill of materials.